The present invention relates to a method and a reference system for monitoring changes in a fluid. Reference electrodes providing a constant reference potential, for example for pH or redox measurements, have been previously known also in the form of so-called electrolyte bridge systems comprising two diaphragms arranged in series (German Patent Publication No. 32 03 406, German Patent Publication No. 32 03 407). Such reference electrodes consist preferably of a silver/silver chloride electrode immersed in a solution of a constant chloride ion concentration. In the simplest of all cases this solution, which may also be gel-fixed, is connected with the measured solution via a diaphragm. It is, however, problematic in such reference systems that they are not constant as regards their potential because of the possibility of contamination from the outside which can never be excluded. Diffusion may occur through the diaphragm in both directions, due to concentration differences between the reference electrolyte and the measured solution, and when measured solution migrates into the interior of the reference electrode, there is a risk that the diaphragm may be obstructed due to crystallization and that the before-mentioned change, namely contamination of the reference electrolyte and the derivation system may occur. In the opposite direction, a loss of reference electrolyte or changes in concentration of the electrolyte due to so-called ablution may occur. In this connection and in supplementation of the generally accepted concept of diffusion, hydraulic movement of the medium through the diaphragm must be considered also in view of the required hydraulic permeability of the diaphragm or diaphragms, the before-mentioned movements being caused by pressure differences resulting from over or underpressure of the measured solution relative to the pressure of the reference electrolyte, or simply from variations in temperature entailing changes in the differential pressure. Another possibility consists in sealing the reference electrode off hermetically against the outside and filling its interior completely so that any external pressure variations of the measured solution cannot provoke hydraulic movement through the diaphragm. In this case, however, problems may be encountered as regards the necessary highly precise filling in which under certain circumstances air bubbles may be trapped so that no sufficiently high counterpressure can build up in the reference system to counteract the varying external pressure of the measured solution. Moreover, when the reference electrode is sealed off hermetically, hydraulic movement through the diaphragm may be obtained already as a result of temperature variations which in the case of repeated substantial temperature differences may lead to something like "breathing" of the enclosed volume, and this again results in hydraulic migration through the diaphragm in both directions, even in the hermetically sealed system.
Finally, it is a drawback of the known reference systems that the risk of contamination of the reference electrolyte and the derivation system can never be excluded completely, not even in systems providing optimum protection against diffusion, and that the user has no means to know whether or not and to what extent this is actually the case in the system operated by him and whether or not maintenance, exchange or adjustment is required.
Now, it is the object of the present invention to improve a reference system for use in analytical measuring techniques, that may already have been optimized as regards diffusion and pressure difference and which comprises two diaphragms arranged in series, so as to render it capable of indicating automatically its condition and insofar also any need for maintenance or exchange, and of providing further the facultative possibility to ensure that any exchange of fluid through the diaphragm, in particular under the aspect of hydraulic movement caused insofar by over or underpressure of the measured solution and/or by temperature changes, is practically excluded.